Synthesis of polyethylene (PE) in solution polymerization is achieved using transition metal based catalysts, namely titanium tetrachloride (TiCl4) and vanadium oxy trichloride (VOCl3) and co-catalyst such as Triethylaluminium (TEAL, Al(C2H5)3). During the polymerization process the co-catalyst gets converted to Al(C2H5)2Cl,Al(C2H5)Cl2 and finally to AlCl3. Some by-products like butene-2, octene-2 are also formed during polymerization due to isomerization of co-monomers like butane-1 and octane-1. Deactivation substances are also added to prevent isomerization. Deactivation of the catalyst is carried out after polymerization to terminate the polymerization reaction and to maximize the polymer product color. Organic fatty acids (C8-C10), such as n-nonanoic acid and the like, are used as deactivators.
Deactivated catalyst residues as well as deactivator material are removed by passing the polymer solution through a fixed adsorbent bed called solution adsorber loaded with activated alumina at operating conditions of pressure and temperature like 120-150 kg/cm2 and at 220-300° C.
Activated alumina is used as the adsorbent of choice to remove the catalyst and co-catalyst residues present in the form of chlorides like TiCl3, VOCl2/VOCl organometallic complexes (catalyst residues) and Al(C2H5)2Cl coming from the unreacted or deactivated catalyst and carboxylic acid from deactivator C8-C10 organic fatty acids.
Activated alumina used for adsorptive removal of catalyst residues (organometallic complexes) is porous gamma alumina. Activated alumina performs adsorption both by physisorption as well as chemisorption. Activated alumina is chosen as the adsorbent, not only because of its porous structure and high surface area, but also because of the availability of acid and base sites as Lewis and Bronsted sites on its surface that play an important role in the removal of deactivated catalyst residues, especially in the case of ethylene copolymers.
Isomerization of the comonomer butene is another parameter for adsorbent selection. Lewis acid sites available on the alumina surface are responsible for isomerization of 1-butene to 2-butene (cis and trans) at high operating temperature of 280° C. Isomerization is expected to be severe at the start of the adsorber bed operation and is further impacted by hold up time of the comonomer with the alumina. Activated alumina is treated with sodium salts during commercial manufacture to minimize isomerization. Optionally, low concentration of sodium salts doping up-to 3 wt % not only reduces the isomerization effect but also enhances the color of the polyethylene resin product.
Crush strength of the fresh and spent alumina is an indicative element of the resistance of the alumina to compaction of the adsorber. Higher the crush strength better is the ability to withstand the effect of melt index changes of the polyethylene mixture. Attrition loss is indicative of the amount of alumina that is expected to be lost by the alumina element rubbing together. Fines resulting from attrition tend to create undesirable fish eyes in the polymer as well channeling in the adsorber bed.
Activated alumina adsorbents used for treating polymer solutions for removal of deactivated catalyst residue are associated with problems like formation of dust due to lower crush strength and attrition leading to high pressure drop across the bed ultimately affecting the resulting throughput. Most of the organometallic residues being relatively larger sized complexes are adsorbed only on the surface with the core of the alumina being left unutilized. This phenomenon is compensated by using smaller sized alumina balls which in turn may lead to higher pressure drop across the adsorber on account of lower crush strength and increased attrition.
No remedy to the above problem has been reported in the literature, it is therefore desirable to have an adsorbent having superior adsorption of deactivated catalyst residues.
A composite alumina-zeolite adsorbent is disclosed in U.S. Pat. No. 3,899,310 which is claimed to adsorb fatty acid compounds from refrigerant gases. The composite adsorbent takes up about 30 mg. of fatty acid per gram of adsorbent, while alumina takes up about 21 mg. of fatty acid and zeolites about 9 mg. of fatty acid per gram of adsorbent indicating superiority of the disclosed adsorbent composite over individual components. It is also suggested in the disclosure that alumina-zeolite composites can be substantially strengthened by moisture treatment.
A method of preparing alumina-based bodies suitable for use as adsorbents or catalysts is disclosed in U.S. Pat. No. 5,935,894 to provide composite bodies either by spraying or soaking the aqueous solutions containing the alumina powder and the alkali metals and the soluble carboxylic acid. Thereafter, the bodies are thermally treated to activate alumina therein and remove or decompose the organic part, e.g., carboxylic acid, of the soluble carboxylic acid alkali metal salt.
U.S. Pat. No. 4,762,537 discloses the use of an adsorbent comprising zeolite Y and alumina to remove HCl from hydrogen gas. U.S. Pat. No. 6,013,600 discloses the use of alumina with alkali and alkaline earth metals for removing HCl and other contaminants. Composite solid shaped adsorbents comprising an alumina, zeolite and metal component for removal of acidic impurities and organic compounds from ethylene stream are disclosed in U.S. Pat. No. 66,632,766.
Canadian patent 732279 discloses use of activated alumina for removal of deactivated catalyst residues from polymerization reaction in a solution polymerization process.
An object of the present disclosure is to provide an attrition-resistant, shaped, solid, composite adsorbent for adsorption of deactivated catalyst residues and deactivation materials from polyolefin in the solution polymerization technology under critical process conditions of temperature and pressure.
A related object of the disclosure is, therefore, also to provide a method for producing said composite adsorbent with additional properties like porosity & lower surface reactivity.